1. Field of the Invention
The present invention relates to a method for measuring a refracting power of an optical system such as lenses for glasses and a contact lens, and its apparatus, particularly to a method for measuring a refracting power of an optical system and its apparatus having a basic principle of retinoscopy, capable of easily measuring a refracting power of an optical system with high accuracy.
2. Discussion of the Background
Conventionally, as shown in Japanese Unexamined Patent Publication No. 280544/1986, Japanese Unexamined Patent Publication No. 46130/1988 and the like, a kind of an apparatus for measuring refracting powers in various optical systems such as in a lens for correcting abnormality of refraction of an eye and lenses for optical instruments, is known which is provided with the basic principle of retinoscopy, wherein the refracting power of such an optical system to be inspected is measured by projecting a predetermined luminous flux to an optical system to be inspected and by detecting a beam transmitted through the optical system to be inspected by a beam receiving element for measurement, based on the moving direction, the velocity and the twist of the transmitted beam.
In the conventional apparatus for measuring a refracting power, to detect the moving direction, the velocity and the twist of the luminous flux transmitted through the optical system to be inspected by the beam receiving element for measurement, a rotating drum or a rotating plate provided with slits having at least two kinds of angles of inclination with respect to a rotational direction at its edge portion, is disposed on an optical path for intermittently blocking the luminous flux and photoelectric converting elements are disposed as the beam receiving elements for measurement at positions forming at least two pairs thereof, centering on an optical axis, and the refraction power of an optical system to be inspected is provided from a difference of phase in outputs of the respective paired photoelectric converting elements.
However, in the apparatus for measuring refracting power having such a construction, it is necessary that the angles of inclination at the edge portion of the slit provided on the rotating drum or the rotating plate and the positions for providing the respective paired photoelectric converting elements provided at the beam receiving element for measurement correspond to each other, and they can not be determined independently. Therefore, there is no degree of freedom in design and the fabricating and adjusting thereof are difficult.
Furthermore, in using the rotating drum, since the surface of the drum is of a curved face, it is extremely difficult to make the edge portion of the slit substantially a straight line with respect to the rotational direction. In using the rotating plate, it is difficult to make constant the angle of inclination of the edge portion of the slit with respect to the rotational direction, in the radial direction. Therefore, it is extremely difficult to maintain the correspondent relationship between the positions for providing photoelectric converting elements at the beam receiving element for measurement and the angle of inclination with high accuracy, over the whole length of the edge portion wherein the position for intermittently blocking the luminous flux is changed in accordance with the refracting power of the optical system to be inspected. Accordingly, it is difficult to sufficiently provide the measurement accuracy.
Furthermore, in the apparatus for measuring a refracting power, the refracting power of the optical system to be inspected is provided from the phase difference of the outputs of the respective paired photoelectric converting elements, on a premise wherein a moving velocity of the luminous flux intermittently blocked by the slits, or a rotational speed of the rotating drum or the rotating plate is maintained constant. Actually, it is difficult to always maintain the rotational velocity of the rotating drum or the rotating plate as constant. A measurement error is apt to be caused by a nonuniformity of rotation.